A variety of blasting methods are known which comprise (1) feeding abrasive particles and liquid carrier into a reservoir and mixing same uniformly therein to form a slurry, (2) sending the slurry to the blasting gun by means of a slurry pump, or ejector means using compressed air, and (3) ejecting the slurry from the blasting gun against the workpiece. These methods include the wet blasting method, the liquid honing method and the water jet method. These blasting methods are widely used for various industrial applications. For example, they are used for descaling of steel fabrications and steel structures, cleaning of various metal surfaces, surface preparation before painting and plating, deburring of machine parts after machining, and so on.
Among the many kinds of known blasting apparatuses, mention is made of the blasting method described in Japanese patent publication No. 29-4747 (1954). In that method (1) compressed air is introduced into the pressure vessel containing the slurry, (2) the slurry contained in said vessel is ejected from the outlet of said vessel, (3) the slurry under pressure is fed to the blasting gun, and (4) the slurry is blasted from the blasting gun so that the abrasive particles and the carrier liquid strike the workpiece. In Japanese patent publication No. 29-4747, the energy of compressed air alone is used to eject and to accelerate the slurry. This is in contrast to other kinds of wet blasting machines, such as the suction-type wet blasting apparatus, or the liquid honing apparatus, in which a slurry pump is used to send the slurry to the blasting gun. Accordingly, the blasting effect was expected to result in lower consumption of compressed air compared with other wet blasting methods.
U.S. Pat. No. 3 343 304 discloses a wet blasting apparatus in which the slurry in the pressure vessel is agitated by means of a rotatable vertical screw.
In the blasting apparatus mentioned in Japanese patent publication No. 29-4747, the blasting operation begins with introduction of compressed air into a pressure vessel after a certain amount of slurry has accumulated in the vessel. Then the pressure of the compressed air on the slurry causes the slurry to be discharged through the outlet of the pressure vessel. Consequently, the duration of the blasting time is limited by the amount of abrasive particles and carrier liquid contained in the vessel. A continuous operation for a long time is difficult. This is one defect of this method.
The duration of the wet blasting operation is limited, and its length is determined, by the volume of the pressure vessel. If the volume of the pressure vessel is increased to provide a longer duration of blasting, another problem arises. It will then be difficult to provide a uniformly mixed slurry. Moreover, as the particle size of the abrasive becomes larger and/or the abrasive particles become heavier, the more difficult it becomes to mix the abrasive particles and the carrier liquid to obtain a uniform homogeneous state and to maintain the resulting slurry in that state. In practice, the range of usable abrasive particle size and the range of duration of blasting time to obtain an even blasting effect are limited in this system.
Because of the above-mentioned limitations or defects of the pressurized wet blasting method using a pressure vessel, the pressurized wet blasting method is used in only a limited range of applications.
Japanese patent publication No. 31-9747 (1956) discloses a method in which abrasive particles and carrier liquid (water) are mixed within a pressure vessel. A perforated steel plate is provided at the bottom of the vessel. Pressurized water is fed in the bottom of the vessel and this water stream flows upwardly to the upper part of the vessel through holes in the perforated steel plate. The water and the abrasive particles are mixed immediately above the perforated steel plate. In this apparatus, the amount of abrasive particles and water in the vessel is limited so that blasting cannot be conducted for a long time, uniformly and continuously.
It is an object of the present invention to provide an apparatus of the pressurized blasting system type in which the consumption of compressed air is minimized, and the blasting operation can be performed continuously and uniformly.